Many vehicles are equipped with four-wheel-drive systems. In some systems, generally referred to as on-demand 4-wheel drive systems, substantially all engine torque is supplied to a single axle during ordinary driving, and torque is transferred to the other axle when necessary. For instance, engine torque is supplied to the rear axle for ordinary driving, but when the rear wheels begin to slip on a low-traction surface, at least some of the engine torque can be supplied to the front axle to maintain vehicle stability.
When the vehicle turns, the front wheels move faster than the rear wheels. If torque is being split between the front and rear axles, the vehicle will resist turning, especially on concrete or other high-traction surfaces. Also, stress can be induced in the drive train and other components while turning.
Thus, four-wheel-drive systems were developed that allow engine torque to be distributed between the front and rear axles and yet allow for speed differences between the front and rear axles while the vehicle is turning. Active four-wheel-drive systems of this type rely on steering angle sensors to detect when the vehicle is turning. These steering angle sensors can be relatively expensive. In the absence of a steering angle sensor, these systems allow for large speed differences between the front and rear axle to allow vehicle turning. However, there is typically a significant delay before torque transfer, and transfer is abrupt and produces undesirable noise. Accordingly, there remains a need for a less expensive and smoother operating on-demand four wheel drive system.